The reliable performance of a propellant driven munition system is dependent on the performance of its propulsion unit. The ignition train has historically been the weakest link in the repeatability of the munition system. Coupling the initiation of the ballistic cycle through various intermediate steps until a smooth transition to full combustion is achieved is the key to reliability.
In a mortar system, the mortar propulsion system is initiated by either an electric or a percussion element in the primer head, which in turn ignites a black powder pellet(s) (igniter) in the base of the flashtube. The combustion products (including gases) are then vented through flash holes in the flashtube, igniting the propellant in the mortar ignition cartridge. The combustion products in the mortar ignition cartridge then vent through holes in the mortar boom. Gas pressure from the combustion products starts the motion of the mortar projectile through the launch tube and the pressure and temperature of the products ignites any additional propellant increments attached to the boom.
Dynamic and hydrodynamic processes occur during the ignition and combustion phase of the launch. The black powder pellet is seated on a small shoulder at the base of the flashtube. Once the black powder pellet starts to be consumed by the combustion process, its diameter starts to reduce and it becomes free to travel within the flashtube, driven by local pressure gradients.
Thus, the black powder pellet becomes free to oscillate within the flashtube, covering and uncovering flash holes in the flashtube and creating pressure oscillations within the mortar ignition cartridge. As combustion is driven by the local pressure that the propellant experiences, the oscillations within the flashtube can generate pressure waves within the mortar ignition cartridge, which can propagate even further into the main combustion chamber.
Since pressure is the driving force of the flashtube venting, it can cause the steel pin end of the flashtube to more vigorously initiate local combustion. This can cause further pressure waves within the mortar ignition cartridge, which can be transferred to the combustion in the launch tube.
At low zones of propelling charge, which has the lowest loading density of propellant in the mortar system, the effect of the waves may be minimal. At higher zones and loading densities, the pressure waves can cause erratic performance and, in the most severe instances, break fins off the mortar boom. This in turn causes an inaccurate trajectory and at worst a safety problem.
As a result, present mortar systems can have severe pressure waves in the flashtube that transfer and grow in the mortar ignition cartridge and the launch tube.
What is therefore needed is a nozzle for use in the flashtube of the mortar launch unit, in order to improve the performance of the mortar. The nozzle should create more homogeneous combustion among the black powder pellet(s) in the flashtube. The nozzle should result in a decrease in the variation, in terms of mass flow rate, of venting products from the flashtube vent holes. The nozzle should create a more consistent muzzle velocity of the mortar. The nozzle should result in a decrease in pressure waves within the flashtube and therefore within the mortar ignition cartridge. Prior to the advent of the present invention, the need for such a nozzle has heretofore remained unsatisfied.